With a one-touch connection pipe joint, simply inserting a plug (male joint member) into a socket (female joint member) causes a spherical locking element of the socket to engage with a locking recess formed on the outer surface of the plug inserted, thereby locking the pipe joint to prevent the plug from being pulled out of the socket. There have heretofore been developed some different types of one-touch connection pipe joints.
FIGS. 6 and 7 show a pipe joint 100 of one of the above-mentioned types. The pipe joint comprises a socket 101 and a plug 105 that is inserted into the socket. The socket 101 has a cylindrical socket body 110, a collar 130 installed in the socket body 110 slidably in the longitudinal direction of the socket body, and a sleeve 120 provided around the outer periphery of the socket body 110 displaceably in the longitudinal direction and urged forward of the socket body 110. When the plug is not inserted in the socket, the collar 130 is spring-urged to a forward position, and a locking element 114 is pushed radially outward by the collar and received in a forward recess 131 of the sleeve to hold the sleeve in a rear position against spring urging. When the plug is inserted into the socket, the plug is inserted into the collar, and the forward end surface of the plug presses and deforms an O-ring 133 provided on the forward end surface of a movable valve member 132 integrally formed with the collar 130. At the same time, an annular locking element-pushing portion 152 provided on the outer peripheral surface of the plug substantially engages with the forward end surface of the collar 130. While doing so, the plug pushes the collar rearward. When a locking recess 153 provided on the outer peripheral surface of the plug radially aligns with the locking element 114, the locking element enters the locking recess 153. Consequently, the sleeve 120 is displaced forward to hold the locking element in the state of being pressed in the locking recess, thereby locking the plug.
The above-described conventional pipe joint has the following problems.
That is, if the plug 105 is inserted obliquely as shown in FIG. 6, the forward end of the plug 105 may come in contact with the forward end of the collar 130, causing a part of the collar 130 to be pushed undesirably. This may cause the movable valve member 132, which is integrally formed with the collar 130, to be displaced from the position of sealing engagement with a fixed valve member 117 provided at a position along the axis of the socket body, resulting in cancellation of the sealing of the socket, and causing leakage.
In addition, if the plug 105 inserted into the socket is suddenly pulled out for some reason after the collar has been pushed in to a position where the forward end of the collar disengages from the locking element, the locking element 114 may be slightly displaced radially inward by the sleeve 120 before the collar is returned forward by the spring force, resulting in only the plug being pulled out of the socket, with the collar being left unable to return. In this state, the movable valve member also has not yet returned to the position of sealing engagement with the fixed valve member. Therefore, there will be fluid leakage from the socket.
Patent Literature 1 noted below proposes a pipe joint for high-pressure fluid that has two locking elements for locking the plug, in which one of the locking elements is configured to function as a sleeve-operating element.    Patent Literature 1: JP-012693Y
However, the pipe joint according to the above-mentioned proposition has not yet solved either of the above-described problems because its purpose is not to solve the foregoing problems.